Method and apparatus for determining the integrity of a solid propellant rocket motor



3,181,245 METHOD AND APPARATUS FOR DETERMINING THE INTEGRITY May 4, 1965 L. w. JENKINS ETAL OF A SOLID PROPELLANT ROCKET MOTOR 3 Sheets-Sheet 1 Filed Sept. 27, 1961 Lonnie W. Jenkins,

Thomas H. Pm,

INVENTORS.

5. 20% AT, 'p bmd'f Lam Fm ATTORNEYS.

Filed Sept. 27, 1961 L. W. JENKINS ETAL. METHOD AND APPAR 3,181,246 ATUS FOR DETERMINING THE INTEGRITY OF A SOLID PROPELLANT ROCKET MOTOR 5 Sheets-Sheet 3 FIG. 4

Lannie W. Jenkins,

Thomas H. Pmi"? INVENTORS.

BY 3. (J. 220W AT'PM MW LFW ATTORNEYS.

United States Patent METHOD AND APPARATUS FOR DETERMINING THE INTEGRITY UP A SOLID PROPELLANT RGCKET MGTUR Lonnie W. Jenkins and Thomas H. Pratt, Huntsville, Ala., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Army Filed Sept. 27, 1961, Ser. No. 141,228 6 Claims. (Cl. 33-174) This invention pertains to an apparatus and method for determining the integrity of a solid propellant motor and more particularly to an electro-rnechanical device for nondestructive testing of case-bonded solid propellant motors.

A case-bonded solid propellant motor generally consists of a circular metal case closed at the head end. A mixture of a polymerizable monomer and a solid oxidizer is poured into the case; the monomer is polymerized, forming a single grain and is bonded to the metal case. A mandrel, inserted at the center of the motor prior to the polymerization, forms a perforation along the longitudinal axis of the motor. This perforation may be circular, star shaped or any of other desired configurations; however, when the mandrel is removed, a perforation of the same configuration as the removed mandrel is left in the solid propellant and thereby forms the burning surface of the propellant when the propellant is ignited.

Several defects are possible in case-bonded propellant grains. Some such defects are: longitudinal cracks, eX- tending from the perforation toward the motor wall; transverse cracks, also extending from the perforation tothe wall; delarnination in which the propellant separates from the motor wall; and the presence of inhomogeneities within the propellant which do not extend to the surface.

The above mentioned defects result in an increase in the surface area or burning area and, upon ignition of the propellant or shortly thereafter, a corresponding increase of pressure within the motor casing which could result in explosion of the motor.

Various methods have been utilized in an attempt to reveal the above mentioned defects. Radiographs have met with little success since the difference in opacity of cavities and propellant is very small. Various ultrasonic methods have been used but again without notable success since cavities and propellants have similar wave transmission characteristics.

One successful solution to the problem of revealing structural flaws in solid propellant grain seems to lie in the actual measurement of the perforation of a propellant grain.

The soundness of this solution to test a propellant grain becomes apparent when one recognizes that case-bonded solid propellants shrink away from the mandrel during polymerization of the monomer and, since the propellant is constrained by the case-bond at the motor wall, strains are set up which can be relieved only by cracking or by separation of the case from the wall.

At its free ends, the propellant can relieve strains to a certain extent by pulling in from the free ends. The internal diameter of the perforation at these points will, therefore, be somewhat less than the diameter of the perforation further removed from the ends.

For each propellant composition, motor diameter, motor length, grain temperature, and perforation configuration, there will be a characteristic perforation shape and such a perforation shape can be determined experimentally. This characteristic perforation shape can then be used in comparison with the measurements of the motor being tested.

Any flaws within the propellant grain will be manifested by a reduction in the perforation diameter, and foreign ice objects present in the propellant will cause a variation from the normal shrinkage of the grain which may be detected by physical measurement of the profile of the perforation.

It is, therefore, an object of the present invention to provide an apparatus and method to detect flaws in solid propellant grains of rocket motors.

A further object of the present invention is to provide an apparatus and method for non-destructive testing of solid propellant motors.

The apparatus of the present invention utilizes transducers which translate mechanical movement into electrical potential so as to measure deviations in the profile of a perforation in a solid propellant motor for detection of any flaws therein. The transducers act as differential transformers and have spring loaded feeler balls attached thereto. The transducers and feeler balls are mounted on a trolley and the trolley is in turn mounted on a beam placed concentrically in the motor perforation.

The trolley is moved along the beam at constant velocity and the spring loaded feeler balls contact the surface of the propellant for slideable movement therealong simultaneous with the movement of the trolley. A reciprocal movement of the feeler balls results as the balls ride over any irregularities in the propellant surface and this movement is converted to an electric signal. The signal is fed through an amplifier to a strip chart recorder to give a trace which is a representation of the profile of the grain.

Other objects and advantages of the present invention will becomereadily apparent to one skilled in the art from the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a perspective view of a propellant motor with the profilometer used in the testing thereof and with a portion of the motor casing being broken away.

FIGURE 2 is a perspective view, partially in section, of the profilometer.

FIGURE 3 is a sectional view of the transducer taken along its center line, and showing the manner in which it is assembled to the trolley.

FIGURE 4 is a longitudinal sectional view through the propellant motor showing the profilometer disposed therewithin of the assembly.

FIGURE 5 is a schematic diagram of the transformer as used in the profilometer.

As shown in FIGURE 1, a profilometer assembly 10 is placed along the longitudinal axis 12 and in the perforation 14 of a rocket motor 16 to test the integrity of the propellant 18 thereof.

The profilometer assembly 10 includes a pair of mechanical to electrical transducer and feeler ball assemblies 20 secured to a trolley 22 which in turn is mounted on a beam or frame 24 for slidable movement thereon.

As shown in FIGURE 4 the frame 24 is rotatably mounted between an end plug 26 in the forward end 35 of motor 16 and a mounting plate 28 at the aft end 29 of motor 16.

To permit the frame 24 to be rotated for successive readings of the valleys of the star shaped perforation there is provided an end bracket 27 which is removably secured to frame 24 and rotatably secured to mounting plate 28 which is in turn secured to end 29 of motor 16. A clearance 31 is provided between the end 29 of motor 16 and mounting plate 28. The trolley 22 is withdrawn into the clearance 31 and rotated therein and reinserted into the perforation in the next valley to be tested. Adjustable set screws 30 are provided in bracket 27 to permit adjustment and centering of the profiloineter 10 in the perforation 14 of motor 16.

The support frame 24 may be a beam as shown in FIGURES 1, 2 and 4 or any other suitable support means. However, as shown in FIGURE 2, the support beam is preferably provided with Teflon strips 32 thereon to permit ease of movement of trolley 22 and serves to electrically isolate the metal trolley 22 and frame 24 one from the other and' to eliminate the possibility of the two metal parts coming into frictional engagement and possibly igniting the propellant during the movement of the trolley along the beam.

The trolley 22 is comprised of a plurality of plates 21 secured around the frame 24 for slidable movement thereon. Trolley 22 further includes brackets 33 thereon to secure a connection 34 thereto and permit electrical connection to the transducer assemblies 20. Trolley 22 further includes a pair of brackets 23 and each of the brackets is provided with a hole 25 therein to secure the transducer assembly to the trolley 22 in a manner which will presently appear.

Each of the transducer assemblies 20 (FIGURE 3) is comprised of a linear differential transformer 36 whose electrical output is a function of a movable core 38 therein. A Teflon feeler ball 40 is threadably secured to a rod 42 at one end 44 thereof and the rod 42 is threadably secured at its other end 46 to one end 4-8 of the movable core 38 so as to be in coaxial alignment therewith. A second rod 59 is threadably secured at one end 53 to the other end 51 of the movable core 38. Rod 50 is provided with an axial aperture 52 therein opening through the other end thereof. A member 54 having one end 71 threadably secured to an end plate 70 disposed at one end of the housing 56 of the tran ducer assembly has its free end 58 extending into the aperture 52 to serve as a guide for the rod 50 when the core 38, rod 42 and rod 50 are moved as a unit. A sealed housing including coils 60 is disposed within housing 56 and arranged around core 38. The coils 60 are adapted to be electrically actuated through a first set of conductors 62 to set up a magnetic field within the transformer 36 in the operation of the transducer assembly. A second set of conductors 64 is connected from coils 60 to an amplifier 66 which is in turn electrically connected into a strip chart recorder 68 to transmit electric signals thereto from the coils 6%.

To pivotally secure the transducer and feeler ball assembly 20 to trolley 22, the housing 56 is provided with the end plate 70 threadably attached at one end 69 thereof and the end plate 70 is provided with a pair of drilled holes 73 to be aligned with the threaded holes 25 of brackets 23. The transducer assembly 20 then is secured to trolley 22 by passing screws 27 in threaded engagement with holes 25 and into holes 73 of end plate 70. At the other end 72 of housing 56, a second end plate 74 is threadably secured thereto (FIGURE 3).

An axial aperture 76 is provided through the plate 74 to permit the rod 42 to extend therethrough. A rod guide tube or member 78 is threadably secured within aperture 76 to permit rod 42 to extend through its axial bore 89.

To permit vertical reciprocal movement of the Teflon ball 4%, a spring 82 is disposed within bore 80 and around rod 42 and seats at one end on a shoulder 84 provided within the ball 4%) (FIGURE 3). The other end of spring 82 abuts against a shoulder 86 formed within bore 80 of member 78.

To move the transformer and ball assembly 20 on frame 24 along the surface of propellant 18, there is provided motion transmission means including a reversible motor 88 connected to trolley 22 by means of gears 90, 92 and 94 (FIGURE 2). Gear 90 is attached to the output shaft of motor 88 and meshes with gears 92 and 94 fixedly carried on the outer ends of right hand threaded shafts 96 and 98. Motor 88 may be mounted on plate 100 secured to frame 24 as shown in FIGURE 4. The threaded shafts 96 and 98 are preferably made of non-sparking material.

Limit switches 102 (FIGURE 2) placed at the ends of frame 24 are electrically connected to motor 88 to reverse or stop motor 88 as trolley 22 reaches the end of its travel in each direction and thereby permitting repetitive scanning of the surface of the perforation 14.

In operation, the profilometer assembly 10 is mounted concentrically in the perforation 14 of the rocket motor 16. Constant input voltage is then applied to coils 60 of transformers 36 from a source of current (not shown) through conductors 62 and a uniform magnetic flux is induced throughout coils 60. The reversible motor 88 is actuated, causing shafts 96 and 93 to rotate and move trolley 22 along frame 24 to carry transducer and ball assembly 215 therewith.

The spring loaded feeler balls 49 are urged by springs 82 into contact with the surface 14 of the propellant 18. Any irregularities on the surface 1'4 of propellant 13 will cause a reciprocal movement of the spring-loaded balls 40 as they ride over these irregularities. Each ball 40 is secured to the rod 42 which in turn is secured to the core 38 centered within coils 66). Therefore, any movement of ball 49 will cause a corresponding movement of core 38.

As shown in FIGURES 3 and 5 each differential transformer consists of the soft iron core 38 and a three winding coil 66. The primary coil 59 is fed from an external alternating current excitation source (not shown) through conductors 62. The two secondary coil windings 61 and 63 are connected in series-opposing. When the core 33 is at center there is an equal voltage induced in each of the secondary windings 61 and 63 and there is therefore zero output from the secondary circuit. However, when the core 38 is moved away from center due to reciprocation of balls 40, more voltage is induced in one secondary winding than in theother, and this gives an output voltage which represents the core displacement and its direction. Thus, a signal voltage is generated and transmitted by means of conductors or to an amplifier 66 and then to a strip chart recorder 68 which gives a representation thereon of the profile of the propellant grain. This representation can then be compared to a trace of an ideal configuration representation and an accurate determination made as to the integrity of the propellant as it exists in the motor at the time of testing in the above described manner.

What is claimed is:

1. In a solid propellant motor having a longitudinal perforation therein, a profilometer for tracing the profile of a solid propellant grain along the surface of the perforation for testing the intgerity of said motor comprising; a frame disposed along the longitudinal axis of said perforation, a trolley mounted on said frame for movement therealong, motion transmission means carried by said frame and connected to said trolley for movement thereof, a reversible electric motor connected to said motion transmission means to effect movement of said trolley responsive to actuation of said motor, transducers carried by said trolley for movement therewith, contact means carried by said transducers for reciprocal movement normal to the path of movement of said trolley, resilient means urging said contact means into engagement with the surface of said perforation to trace the profile of said perforation and said transducer means generating a signal corresponding in amplitude to said reciprocal movement of said contact means, receiving means for recording the signal generated by said transducers, and means connecting said transducer means to said receiving means.

2. A profilometer as in claim 1 with said motion transmission means including a pair of threaded shafts journaled on said frame and threadably engaging said trolley, each of said shafts having a gear at one end thereof, said electric motor having a gear secured thereto and in mesh with said gears of each of said shafts to cause rotation thereof and thereby move said trolley along said frame responsive to the actuation of said electric motor.

3. A profilometer as in claim 1 with said frame having limit switches secured thereto and electrically connected to said electric motor, said trolley disposed to contact said switches for reversing the operation of said electric motor to permit repetitive scanning of said propellant by said profilometer.

4. A profilometer as in claim 1 having a rotatable end plate secured to said frame and said rocket motor, adjusting means secured to said end plate and connected to said frame for centering said frame in axial alignment with said rocket motor.

5. A profilometer as in claim 1 with said frame having insulation strips disposed between said trolley and said frame.

6. A profilometer as in claim 1 with each of said transducers comprising a housing secured to said trolley, a plurality of coils having a movable core therein and enclosed by said housing, a first set of conductors connected to said coils and to said source of electric energy for energization of said coils, contact means secured to said core and provided with resilient means for reciprocal movement of said contact means, said coils provided with a second set of conductors for attachment to said receiving means for transmission of electric signals thereto responsive to movement of said core.

Reserences Qited by the Examiner UNITED STATES PATENTS 1,618,804 2/27 Bontempi 3323 X 2,623,293 12/52 Nebesar et al. 33-174 2,812,585 11/57 Broughton et a1 33174 2,833,046 5/58 Jeglum 33172 X 2,859,523 11/58 Polidar 3340 X 3,024,651 3/62 McGlasson 33178 FOREIGN PATENTS 1,194,119 11/59 France.

ISAAC LISANN, Primary Examiner.

LOUIS R. PRINCE, Examiner. 

1. IN A SOLID PROPELLANT MOTOR HAVING A LONGITUDINALLY PERFORATION THEREIN, A PROFILOMETER FOR TRACING THE PROFILE OF A SOLID PROPELLANT GRAIN ALONG THE SURFACE OF THE PERFORATION FOR TESTING THE INTEGRITY OF SAID MOTOR COMPRISING; A FRAME DISPOSED ALONG THE LONGITUDINAL AXIS OF SAID PERFORATION, A TROLLEY MOUNTED ON SAID FRAME FOR MOVEMENT THEREALONG, MOTION TRANSMISSION MEANS CARRIED BY SAID FRAME AND CONNECTED TO SAID TROLLEY FOR MOVEMENT THEREOF, A REVERSIBLE ELECTRIC MOTOR CONNECTED TO SAID MOTION TRANSMISSION MEANS TO EFFECT MOVEMENT OF SAID TROLLEY RESPONSIVE TO ACTUATION OF SAID MOTOR, TRANSDUCER CARRIED BY SAID TROLLEY FOR MOVEMENT THEREWITH, CONTACT MEANS CARRIED BY SAID TRANSDUCER FOR RECIPROCAL MOVEMENT NORMAL TO THE PATH OF MOVEMENT OF SAID TROLLEY, RESILIENT MEANS URGING SAID CONTACT MEANS INTO ENGAGEMENT WITH THE SURFACE OF SAID PERFORATIONS TO TRACE THE PROFILE OF SAID PERFORATION AND SAID TRANSDUCER MEANS GENERATING A SIGNAL CORRESPONDING IN AMPLITUDE TO SAID RECIPROCAL MOVEMENT OF SAID CONTACT MEANS, RECEIVING MEANS FOR RECORDING THE SIGNAL GENERATED BY SAID TRANSDUCERS, AND MEANS CONNECTING SAID TRANSDUCER MEANS TO SAID RECEIVING MEANS. 